In recent years, there are many new developments in the area of AlGaInP light emitting diodes with high efficiency. Among the newest AlGaInP light emitting diodes, many of them are based on a double heterostructure comprising an upper cladding layer, a lower cladding layer, and an active layer between them. In general, the light emitting efficiency is quite limited because of the high resistivity in the upper cladding layer that makes the spread of the electric current difficult. Several techniques and structures have been developed to improve the luminous efficiency for the light emitting diodes. One technique in the commonly used double heterostructure is to grow various types of window layer or layers above the upper cladding layer to help the spread of the current. The other popular approach is to add a current blocking layer above the upper cladding layer to increase the current density and decrease the light absorbed by the electrode.
As shown in FIG. 1, a conventional light emitting diode based on a double heterostructure has an n-type GaAs substrate 12. Below the substrate is an n-type electrode 11. A distributed Bragg reflector layer (DBR) 13 is grown above the substrate first before growing an AlGaInP double heterostructure on the substrate. The double heterostructure includes an n-type AlGaInP lower cladding layer 14, an undoped active AlGaInP layer 15 and a p-type AlGaInP upper cladding layer 16. A p-type thin layer 17 with low energy band gap and high conductivity is formed on top of the double heterostructure. A p-type GaP window layer 18 is then grown above the thin layer 17. Above the window layer 18 is the p-type electrode 19 of the light emitting diode.
Although the electric current can travel uniformly through the active layer of the diode structure described above and the diode can emit light across the whole active area, some portion of the light is blocked by the p-type electrode 19. The blocked light can not emit out of the diode. As illustrated by the arrow lines shown in FIG. 1, the light underneath the electrode can not pass through the electrode. Therefore, the luminous efficiency of the light emitting diode is limited.
U.S. Pat. No. 5,153,889 presents an improvement of the light emitting diode structure developed by Toshiba of Japan to overcome the above drawback. As shown in FIG. 2, the structure includes an n-type electrode 21, an n-type GaAs substrate 22, a lower cladding layer 23, an active layer 24, an upper cladding layer 25, a window layer 26, and a p-type electrode 27. The lower cladding layer 23, active layer 24 and upper cladding layer 25 form an AlGaInP double heterostructure for the light emitting diode. Below the p-type electrode 27 and between the window layer 26 and the double heterostructure, a current blocking layer 20 is formed on top of the double heterostructure. Although light emitting diodes of the above structure can increase their current density and light emitting efficiency, the manufacturing process requires two times of MOVPE process. Because of the difficulty in MOVPE process, in particular, the second MOVPE process is performed on top of the p-type AlGAInP layer that contains Aluminum, the technique is cumbersome and difficult to practice.